1. Field of the Invention
The present invention relates to an anthracene compound and a light-emitting element containing the anthracene compound as a light-emitting substance. The present invention also relates to a light-emitting device, an electronic appliance, and a lighting device each of which includes the light-emitting element.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence (EL) (Patent Document 1 and Patent Document 2). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (a light-emitting layer) is provided between a pair of electrodes. By applying voltage to the element, light emission from the light-emitting substance can be obtained.
Such a light-emitting element is a self-luminous element; thus, a display (a display device) including the light-emitting element has advantages over a liquid crystal display in point of high visibility, no backlight required, and the like. Besides, such a light-emitting element has advantages in that it can be manufactured to be thin and lightweight and has very fast response speed.
Since a light-emitting layer of such a light-emitting element can be formed in the form of a film, planar light emission can be achieved. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, the light-emitting element also has great potential as a planar light source applicable to a lighting device and the like.
In the case of an organic EL element in which a light-emitting layer containing an organic compound as a light-emitting substance is provided between a pair of electrodes, application of voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the light-emitting layer, so that a current flows. By recombination of the injected electrons and holes, the light-emitting organic compound is brought into an excited state to provide light emission.
An organic EL element is known in which an electron-injection layer, a hole-injection layer, an electron-transport layer, and a hole-transport layer are provided between a cathode and an anode for efficient injection of electrons and holes to a light-emitting layer. In such an organic EL element, an anode, a hole-injection layer, a hole-transport layer, a light-emitting layer, an electron-transport layer, an electron-injection layer, and a cathode are generally stacked in this order.
It is known that a small amount of dopant material with high emission efficiency is dispersed in a host material in a light-emitting layer, so that the emission efficiency can be improved. In the light-emitting layer having such a structure, electrons and holes are recombined first in the host material, so that the host material is brought into an excited state. Then the excited energy is transferred to the dopant materials to excite the dopant materials, so that light emission from the dopant materials can be obtained. Such an energy transfer mechanism can improve the emission efficiency of a light-emitting element.
The excited state of an organic compound can be a singlet excited state or a triplet excited state, and light emission from the singlet excited state (S1) is referred to as fluorescence, and light emission from the triplet excited state (T1) is referred to as phosphorescence. The statistical generation ratio of the excited states in the light-emitting element is considered to be S1:T1=1:3. Therefore, a light-emitting element including a phosphorescent compound capable of converting the triplet excited state into light emission has been actively developed in recent years.
An element that emits light in the blue and green regions is most demanded of light-emitting elements containing phosphorescent compounds.